1. Field of the Invention
This invention relates generally to an apparatus for magnetically recording and/or reproducing video and audio signals, which may constitute television signals, and more particularly is directed to improvements in the recording and/or reproducing of the audio signal and specifically to stereo audio signals.
2. Description of the Prior Art
In the case of video tape recorders known in the prior art for recording a color television signal on a magnetic tape, the chrominance and luminance signal components of the color video signal are separated, and the carrier frequency of the chrominance signal is down-converted in relation to the frequency of the luminance component. The luminance component frequency modulates a relatively high-frequency carrier and the high sideband of the frequency-modulated luminance signal component and the frequency-converted chrominance signal are mixed or combined to form a composite video signal that is recorded on the magnetic tape in successive parallel tracks that extend obliquely relative to the longitudinal or running direction of the magnetic tape. These tracks are commonly referred to as "slanted tracks38 . Typically, when recording color television signals in such prior art system the audio signals are not recorded in the slanted tracks but are recorded in a more conventional fashion in a single or double track running in the longitudinal direction of the magnetic tape and are typically referred to as "audio tracks". In the above-described video tape recording system known in the prior art, the slanted tracks containing the frequency down-converted chrominance signal and the frequency-modulated luminance signal are formed by at least two rotary magnetic heads which are adapted to scan alternately the magnetic tape along a path that is oblique to the running direction of the tape. The heads are supplied then with the video signals to be recorded at the appropriate times.
One prior art technique that has been used to increase the recording density of the composite color video signal on the magnetic tape is to eliminate any space between adjacent slanted tracks. Such inter-track spaces are typically referred to as guard bands. Nevertheless, one adverse effect of the elimination of such guard bands is the creation of cross talk between the signals on these closely arranged tracks during reproducing. This problem of cross talk has been solved by utilizing a heretofore undesired aspect of video tape recording relating to azimuth loss, which comes about when the gap of the reproducing head is not aligned with the gap of the head used to record the signal. Thus, by providing the two rotary magnetic heads with substantially different azimuth angles and requiring that each head gap angle must essentially match the azimuth angle of the track being reproduced, a substantial azimuth loss will obtain relative to the high-frequency components of any potential cross talk that is derived from signals recorded in adjacent tracks. Accordingly, cross talk is substantially suppressed in regard to the FM modulated luminance signal. Nevertheless, the azimuth loss phenomenon is not effective with low-frequency signals and, thus, cross talk remains in regard to the frequency down-converted chrominance signal, which has been moved down to a relatively low-frequency band. The prior art involved various measures in attempts to eliminate or minimize the low-frequency component of this cross talk and as disclosed in U.S. Pat. No. 4,007,482 issued Feb. 8, 1977, having a common assignee herewith, such low-frequency cross talk relative to the frequency down-converted chrominance signal component is attenuated by recording the chrominance signal component with different first and second carriers in the adjacent tracks, respectively. Such first and second carriers permit the chrominance signal components to be distinguished from each other and, upon reproduction of the signal recorded in a particular track, the low-frequency band of the cross talk from the tracks adjacent thereto can be suppressed or eliminated. One specific approach disclosed in the above-identified patent involves recording the chrominance signal component of the color video signal with first and second frequency-converted signals having the same carrier frequency in alternate tracks with a constant phase and in subsequent alternate tracks with the phase reversed in polarity for successive line intervals.
This scheme will assure that during playback or reproduction the cross talk effects can be minimized or eliminated. During reproduction of signals recorded in this fashion the two successive line intervals may be added together by means of delay lines, such as embodied by a comb filter. Nevertheless, in view of the above approaches to recording the video portion of a color television signal, the audio signals thereof, as in the case of left and right stereophonic signals, are always supplied to the tape in the running or longitudinal direction by dedicated, fixed heads that are continuously in contact with the magnetic tape to lay down the audio tracks corresponding to the left and right stereophonic signals. As is well known, in magnetic tape recording the bandwidth of the signal that can be recorded is determined to a great extent by the relative velocity between the recording head and the record medium. In regard to recording color video signals, this relative velocity between the tape and the head is provided by the rotational speed of the rotary magnetic heads and, thus, in order to achieve high-density recording without requiring large lengths of tape the transport speed of the magnetic tape is relatively low, for example, a typical tape speed is 1.33 cm/sec. This linear speed of the tape relative to the fixed heads that record the audio signals is quite low, and this results in a reduction in the quality of the audio recording that can be made.
One proposal to increase the quality, that is, the fidelity, of the audio signals in video tape recorders has been to frequency modulate the audio signals then mix the frequency-modulated audio signals with the composite color video signals, with the mixed or combined signals then being supplied to the rotary magnetic heads so that the audio signals are also recorded in the slanted tracks. This then provides a sufficiently high relative velocity between the head and the tape to provide a wide bandwidth for the recorded audio signals. Nevertheless, even this scheme has met with drawbacks because the frequency-modulated audio signals recorded in the next adjacent tracks have the same carrier frequency. Therefore, each audio signal reproduced from a particular track would contain a beat frequency interference due to the audio component of the cross talk from the adjacent tracks. While the level of such cross talk was reduced by the aforementioned azimuth loss phenomenon, the quality of the audio signal was deleteriously affected.
The prior art then proposed a solution to this problem in an improved system for recording video and audio signals in which the audio signal was formed into two FM signals each having different carrier frequencies and different frequency deviation ranges, that is, different locations on the frequency spectrum. In this proposed system the audio signal is formed into two FM signals having different carrier frequencies and different frequency deviation ranges, and the two FM audio signals thus obtained are supplied to the two rotary magnetic heads, along with the processed composite color video signals, for recording in the plurality of slanted tracks formed on the magnetic tape. It is appreciated, of course, that the slanted tracks do not have guard bands arranged between adjacent tracks, and the desired relative isolation of the FM audio signals in adjacent slanted tracks is provided by the different respective carrier frequencies.
While the interference caused by crosstalk between adjacent slanted tracks can be substantially reduced in the reproduction mode of the apparatus described above for converting a single channel audio signal into a pair of FM audio signals and to record the audio signals on the slanted tracks together with the video signal, it has been proposed to use two frequency modulators that operate to frequency-modulate the audio signal with two carriers having different frequencies. Nevertheless, in such a situation it is necessary to construct each frequency modulator so that it has its own individual frequency stabilizing means, such as a phase-locked-loop, in order to obtain a stable FM audio signal that has an accurate carrier frequency. Accordingly, the circuit arrangement to accomplish this, and to obtain the recordation of the FM audio signals, is quite complicated in its configuration, resulting in increased costs of commercial products.
Furthermore, in a reproducing system utilized to reproduce the audio signals from magnetic tape in which each audio signal has been recorded as two FM signals in the slanted tracks without guard bands so that interference caused by cross talk between adjacent slanted tracks is reduced, it has been proposed to demodulate the FM audio signals, which have respective different carrier frequencies and which are obtained respectively from the two reproducing rotary magnetic heads that alternately trace the slanted tracks, by using two individual demodulators that have central frequencies to discriminate the corresponding frequencies of the respective FM audio signals. Nevertheless, in this proposed system for reproducing the audio signals it is required to have two frequency demodulators having respective different central frequencies for discrimination of the audio signal of each channel and, accordingly, the resultant circuit configuration is quite complex. Additionally, another undesirable feature of this proposed demodulation scheme involves a measurable difference that may be present between the frequency-demodulated outputs obtained from the two frequency demodulators, caused by frequency demodulating a single channel audio signal using two frequency demodulators having different frequency discriminating characteristics.